BackgroundInterventions to change the built environment offer a promising approach for promoting physical activity and improving health. However, there has been little theoretical consideration of the underlying mechanisms. In a systematic review we aimed first, to map the potential causal pathways linking environmental changes with changes in physical activity, and second, to review the evidence for specific mechanisms. Here we focus on the first stage of the review. MethodsWe searched seven electronic databases (Cochrane, Medline, ProQuest, HealthEvidence, National Institute for Health and Care Excellence, Transport Research International Documentation, and Web of Science) representing a range of disciplines using a systematic, transparent, iterative process, which was completed in April, 2015. We did not assess study quality or apply date restrictions. Theoretical and conceptual material was extracted from review articles, theoretical papers, and intervention studies. We used thematic analysis in an in-depth, configurative, narrative approach to synthesis, aiming to derive a set of plausible generalisable pathways by which environmental changes could promote changes in physical activity. FindingsOur initial searches identified 2760 potential sources from specialties that included public health, sociology, behavioural science, and transport. We included 79 papers. Broad overarching themes were related to physical activity as a behaviour and a practice, environmental interventions as structural changes, complexity and systems, contexts and observable or unobservable mechanisms. We devised a more generalisable set of hypothesised mechanisms linking environmental changes with changes in physical activity through combinations of changes in both attributes and judgments of both physical and social environments, as well as in individual reasoning. Mechanisms were either conscious or unconscious and operated at the individual and societal levels. InterpretationThis conceptual and theoretical synthesis provides a new generalisable conceptual framework to contribute to the design and conduct of assessments of environmental interventions. Future research should seek to understand judgments and interpretations of the environment and how environmental and behavioural changes become embedded. Our thematic analysis highlights conceptual areas of dissonance and common ground for multidisciplinary research teams. In the second part of the review, we will search for evidence on specific mechanisms to assess what environmental interventions work, for whom, in what contexts, and how. FundingMedical Research Council (unit programme number MC_UU_12015/6). JP was also supported by a National Institute for Health Research postdoctoral research fellowship.